Highly refractive molybdenum bodies and method of preparing same



HIGHLY REFRACTIVE MOLY'BDENUM BODIES AND METHOD OF PREPARING SAMELeonard F. Yntema, Waukegan, Ill., and Edward A. Beidler, Columbus, andIvor E. Campbell, Gahanna, Ohio No Drawing. Application December 13, 1 957 Serial N0. 110,831 v 9 Claims. c1. 148--6.3) I

This invention relates to metal bodies resistant to oxidation at hightemperatures and more particularly to such bodies formed of molybdenumwhich is provided with a coating or skin to render the base metalresistant to oxidation at high temperatures.

This application is a continuation in-part of our copending application,Serial No. 299,216, filed July 16,'

continuous flow of hydrogen is maintained over the' heated resistanceelement's. Molybdenum, of course, is'

desirable for many other applications, such as oil burner nozzles,artillery piece nozzles, rocket nozzles, turbine blades or buckets 'andother component parts of jet engines, ignition coils for gasand oilburners and valve seats forinternalcomhustion engines, the foregoing usebeing merely the best known.

The present invention makes possible the protection of all metals,steel, for example, but particularly molybdenum, which are designed foruse in products which in use are exposed to high temperature, air anderosion.

In the co-pending applications of Campbell et al., Serial No. 150,543,now Patent No. 2,665,997; Serial No. 150,544, now Patent No. 2,665,998;and Serial No. 150,398, now Patent No. 2,665,475, filed March 18, 1950,methods are disclosed and claimed for the production of'refractorybodies'formed of molybdenum having a coating or skin which is resistantto oxidation at elevated temperatures and which ,protects the base orcore from oxidation at such temperatures. The molybdenum bodies formedin accordance with these methods consist of a molybdenum base or corehaving an integral coating or'skin of molybdenurn-silicon alloys orintermetallic compounds. The silicon content of the protective coatingsor skins varies in the molecularv ratio of silicon to refractory metalof from about 1:1 -to about 3:1, which corresponds toyalloys' orintermetallic compounds containing from about22.5% to about-47% silicon.Al-

though the optimum protection ofthe molybdenum isobtained with coatingsor skins having a molecular ratio of silicon tomolybdenum of about 2:1,corresponding to a silicon content of about 37%, coatings or skinsbeyondthe composition range also afford some protection for the molybdenumbase or core. The coating or skin is formed-on the molybdenum.

base by passing a mixture of hydrogen and vapors of a silicon halidesuch as silicon tetrachloride or silicon tetrabromide over the heatedmolybdenum base. molybdenumbase is maintained at a temperature appreciably below the melting point of silicon duringthe Where the2,920,006 Patented Jan. 5, 1960- deposition, the deposited silicon doesnot alloy or dilfuse surface portions of the molybdenum base to form theprotective coating or skin. I

Where the temperature of the molybdenumbody is maintained at atemperature at least as high as the melting point of silicon during thedeposition of the silicon," there is an immediate alloying of thesilicon and the surface portions of the molybdenum. 0

These alloyed coatings or skins on molybdenum furnish anexceedingly highresistance to oxidation at elevated temperatures. 0 For example,'molybdenumwire having a diameter of approximately 0.020" has a life ofapproximately 16 seconds when heated'to a temperature of about 1500 C.in air. The same size molybdenum Wire'When provided with a coatinghaving a thickness of about 0.00032 has a life of 4,000 seconds at thesame temperature.

The thickness of the coating does not represent the total thickness. ofan alloy layer. The total thickness of the alloy layer is roughly aboutdouble the thickness increase which is obtained during the coatingoperation. Throughout this application the term thickness. is used todesignate only the thickness increase effected by the coating operation.

The protection aiforded the molybdenum by the alloy coatings may befurther illustrated by reference to the lives of coated molybdenum barshaving dimensions of about 0.5" x 0.05" x 6" and having coatings of theorder of 0.0015" to 0.002" in thickness. The lives were measured bymaintaining the coated bars at'the designated temperatures'in air. a I

Temperature Life 815 1, 500 In excess of 8,000 hours.

1, 090 2, 000 In excess of 3,000 hours. 1, 370 2, 500 Average abouthours. 1, 650 3,000 Average about 70 hours. 1, 760 3, 200 (1 specimen)77 hours;

1, 850 3,360 Average about 24 hours.

Molybdenum bodies provided with this protectivemolybdenum-silicon;coating or skin, while being a-tre-1 mendous advancein the art, can be made to exhibit two typesot failures.- The first'typeof failureis that which is commonly encountered at lower temperatures,and occurs at thatportion of the body which is maintained at the highesttemperature. For example,- a molybdenum wire secured at opposite ends toelectrodes and heated byIthe passage 'of an electrical currenttherethrough will fail, in general, near its mid-point when maintainedata relatively low. temperature. In such instances the centerportion ofthe wire will attain a higher temperature because ofEthe greater heatloss at the ends of thewire which are secured to terminals. w 3 a Thesecond type of failure which is most commonly en; countered at thehigher temperatures is a failure which occurs 'at a point intermediatean area of maximum temperature and an area'of minimum temperature. 'Itisthus apparent that the life of the molybdenum having the pro tectivecoating or skin is not necessarily and solely de-; pendent upon themaximum temperature to which the body will be subjected. I

The principal purpose of the present invention is to;

provide molybdenum bodies and other metal bodies having a coating or anexterior layer which has an appreciably greater resistance to oxidationat high temperatures than the resistance exhibited by molybdenum-siliconalloy coatings.

A further object of this invention is to provide a highly resistantcoating or skin which does not exhibit a failure at the intermediatetemperature zones.

Another object of our invention is to provide a method of applying to orforming on molybdenum an integrally bonded, highly protective coating orexterior layer which will protect the molybdenum against oxidation athigh temperatures.

Other objects and advantages of this invention will become apparent fromthe description and claims which follow.

The present invention contemplates, in particular, bodies of therefractory metal molybdenum having a coating or exterior layer ofmolybdenum-silicon-boron alloys or intermetallic compounds, but itincludes other metal bodies to which the high temperature,oxidationresistant coating of the present invention is either formedthereon in situ, as by plating the metal base with molybdenum and thenreacting it with silicon and boron, or applied thereto by mechanicalmeans, as by welding, for example. Although we have designated thecoatings of the present invention as molybdenum-silicon-boron alloys orintermetallic compounds, the precise nature of the coatings or skins hasnot been determined. It has been established that in accordance with themethods as described in the aforementioned Campbell et al. applications,alloys or intermetallic compounds are formed between the molybdenum andthe deposited silicon. The inclusion of small amounts of boron increasesthe complexity of the coatings and it appears that the precise nature ofthe coatings is dependent upon the method employed in preparing thebodies. Successive deposition of silicon and boron or boron and siliconproduces a coating or exterior layer having a more complex structurethan the coating or skin produced by simultaneous deposition of siliconand boron. We therefore do not intend to limit our invention to acoating consisting solely of true alloys or intermetallic compounds ofmolybdenum-silicon-boron. Our invention contemplates compositions whichmay consist of such true tertiary alloys or intermetallic compounds orany other type of alloy or composition including molybdenum, silicon andboron in the approximate proportions stated.

We have discovered that the presence of boron in the molybdenum-siliconcoatings or exterior layers as described in the aforementionedco-pending applications vastly increases the protective qualities of thealloy coating and also substantially eliminates the second type offailure described hereinbefore; namely, the failure of the coating at anintermediate temperature zone. The coatings contemplated by ourinvention contain a small amount of boron, that is, an amount of fromabout 2% up to about by weight, preferably between about 3% and about7.5%, based on the weight of the skin or coating.

Boron may be incorporated in our coatings by any desired method. Theboron and silicon may be applied to and alloyed with the molybdenumeither independently or simultaneously.

The boron may be applied to or alloyed with the molybdenum followed bythe application of the silicon in accordance with the aforementionedco-pending applications. V

Molybdenum wire having a diameter of about 0.080" and provided with amolybdenum-silicon-boron skin or coating by the deposition of boron onthe molybdenum followed by the deposition of silicon has an average lifeof about 95 hours when maintained at 1700 C. in air. The maximum lifeexhibited by one of a series of such specimens was 416 hours. Of 20specimens provided with the boron-containing exterior layer, only twoexhibited a failure at an intermediate temperature zone.

when maintained at 1700 C. in air.

Molybdenum-boron compositions in themselves are of little protectivevalue, the life of such coating or skin having a thickness of 0.003"being about 6 hours at a temperature of 1000 C.

The same size molybdenum wire provided with a molybdenum-silicon alloyskin of about 0.001" in thickness has an average life of about 24 hourswhen maintained at a temperature of 1700 C. in air. The maximum lifeexhibited by one of a series of such specimens was as great as 47 hours.I

The simultaneous deposition of boron and silicon produces a coating orexterior layer which possesses greater protective qualities. Coatingsproduced by the simultaneous deposition have exhibited lives in excessof 1,000 hours when maintained at a temperature of 1700 C. in

air.

The application or deposition of the boron and silicon may be effectedby a vapor deposition process and the temperature of the molybdenum bodymay be such-as to promote the alloying action simultaneously with thedeposition. A boron halide or a silicon halide, ora boron halide and asilicon halide, such as the bromides or chlorides, are volatilized andmixed with hydrogen. The halide-hydrogen mixture is then brought intocontact with the heated molybdenum body. The body is main-.

tained at a temperature sufficiently high to cause a reaction betweenthe halide and the hydrogen allowing the boron or silicon, or boron andsilicon to plate out on the molybdenum body. Since it isdesired toproduce what we have termed an alloycoating of molybdenum, boron andsilicon, we prefer to maintain the molybdenum body at a temperaturesufficiently high so as to effect an alloying of or a reaction betweenthe deposited materials and the surface portions of the molybdenumsimultaneously with the deposition of the boron and silicon.Temperatures from about 1400 C. to about 1800 C. are satisfactory. Inthe independent deposition and alloying of. boron and silicon, the boronmay be deposited on and. alloyed with the molybdenum surface bymaintainnig the molybdenum at a temperature of about 1600 C. The siliconis subsequently deposited on and alloyed with the molybdenum-boronsurface at a temperature of about 1800 C. In the simultaneous depositionof boron and silicon a temperature of about 1600 C. is satisfactory.

The protective qualities of the alloy coatings of skins are dependentupon the composition of the coatings or skins. The composition of thecoating or skin is in turn dependent upon the composition ofthe halidehydrogen mixture from which the boron and silicon are derived. As in thecase of the molybdenum-silicon alloys, the

I proportion of silicon in the coatings of this invention alsodetermines the protective qualities of the coatings or skins. Coatingsor skins which exhibit the high protective qualities containcombinations of silicon and molybdenum containing from about 22.5% toabout 47% silicon, particularly about 37% silicon, and combined boron inamounts of about 2 to 10% of the total weight of the coatings.Calculated on a composition basis, the coatings or skins contain fromabout 15% to about 40% silicon, from about 2% to 10% boron, and thebalance molybdenum. The preferred coatings contain about 22% silicon,about 5.5% boron, and the balance molybdenum. The amount of boron in thecoatings has a direct bearing upon the resistance of the coatings ofthis invention. boron deposited is dependent upon the amount of boron inthe halide-hydrogen mixture. The presence of boron in the coatings orskins in amounts exceeding about 10% decreases the life of the coatings.The hydrogen which is employed should be substantially free of watervapor or oxygen. The amount of hydrogen present in the gaseous mixtureshould be at least suflicientto react with the halides and an excessamount is preferably pro- In turn, the amount of' 51 vided to aid in theremoval of the acid vapors which are formed.

We have discovered that satisfactory coatings may be prepared fromatmospheres of gaseous mixtures having molar ratios, between silicontetrachloride and boron trichloride of from about 2.5 :1 to about 18:1and containing from about 1.5 to 6.0 times the amount of hydrogentheoretically required to reduce the halides. Coat.- ings which haveexhibited optimum resistance to oxidation have been obtained fromgaseous mixtures wherein the ratioof silicon tetrachloride towborontrichloride is about 4.621 by volume and having a silicon to boron ratioof about 11.921 by weight. The ratio of the amount of hydrogen in thegaseous mixture to the amount of, hydrogen theoretically required toreduce the halides or react with the halides was about 3.6:1.

In the independent deposition and alloying of boron and silicon withmolybdenum, the molybdenum body is positioned in a suitable chamberwhich is purged with hydrogen. The molybdenum body is then heated tothedesired plating temperature by any suitable means. For example, inthe coating of wire or rod the wire may be heated to a temperature ofabout 1600 C., by passing an electric current therethrough. The hydrogenis passed through a boron halide such as boron tribromide. The boron.tribromide may be maintained at room temperature and the hydrogenpassed through the liquid at the rate of about 800 cc. per minute. Theresulting gaseous mixture is then brought into contact with the heatedmolybdenum wire or rod. Such gaseous flow is maintained for about 5seconds. The flow of gas is then interrupted and the molybdenum bodyheated to a temperature of about 1800 C. Hydrogen is passed through asilicon halide such as silicon tetrachloride maintained at roomtemperature at about the same rate of flow and the flow continued forabout 8 minutes. The gas flow is then interrupted and hydrogen may beslowly passed through the chamber while the coated molybdenum iscooling.

In the simultaneousrdeposition, the coating atmosphere is prepared bypassing hydrogen through a silicon halide such as silicon tetrachloridewhich may be maintained at about C. at the rateof about 800 cc. perminute, and by passing hydrogen through a boron halide such as borontribromide maintained at room temperature at the rate of about 20 cc.per minute. Thetwo gas streams are then merged and brought into contact'with the heated molybdenum which maybe at a temperature of about 1600'C. This gas flow is maintained for about 8 min- Utes.- The gas-vapormixture is then interrupted and hydrogen passed through the containeruntil the coated molybdenum has cooled.

The maintenance of such flow rates and temperatures for the statedperiod will produce a coating having a thickness of the order of 0.002"on a molybdenum wire having a diameter of about 0.080". Molybdenum wireof this size provided with theexterior layer containing occurs atintermediate temperature zones. failure is in general the type offailure encountered where the coated molybdenum body is. maintained attemperatures of about 1200 C. to about 1400" C. and higher. The failuregenerally occurs at a zone where the temperature of the body remains inthe neighborhood of 1200 C. to 1400 0., although the maximum temperatureof other portions of the body may be appreciably higher. The presence ofboron thus permits the coated body to be utilized under conditionswhereby the body attains higher temperatures and protects the body fromfailure at the zones maintained in the neighborhood of about 1300 C. to1500 C. where the body is employed under conditions which result intemperature gradients.

Our invention is not to be limited to the deposition of boron andsilicon on a molybdenum base by a chemical reaction, i.e., the reductionof the halides of boron and silicon by hydrogen, since in accordancewith our in vention the. boron and silicon may be deposited bymechanical means, as by brushing or spraying a suspension of silicon andboron powders on the molybdenum base, or by a metallizing operationwith'the aid of a metal spray gun. No matter how the silicon and boronare deposited, they are caused to react with the molybdenum base by a.heat treatment, as described.

Nor is our invention to be limited to the in situ formation of amolybdenum-silicon-boron skin (layer) on a molybdenum base, since thesaid skin may be preformed and applied either to a molybdenum base or toa base of another metal or alloy such as steel, nickel, titanium, etc.,Which is to be protected from high temperature oxidation, as bymechanical means such as welding and bonding With a metal.

We claim:

1.. The method of forming refractory metal bodies resistant to oxidationat elevated temperatures which comprises heating a molybdenum body to atemperature exceeding about 1400 C., simultaneously depositing boron andsilicon on the body by passing a mixture of hydrogen substantially freefrom water vapor and oxygen and vapors of a boron halide and a siliconhalide over the heated body, and maintainingthe deposited boron and 2.The method of forming refractory metalbodiesre silicon and boron by suchsimultaneous deposition and alloying has exhibited a life of 1415 hoursat 1700 C. (3100 F.) in air and as high as 134 hours at a temperature ofabout 1900 C. (3450 F.).

Increasing appreciably the boron content of the vaporhydrogenatmosphereand thereby increasing appreciably the boron content of thecoating substantially decreases the life of the coating. For example, bydoubling the amount of boron trichloride vapors added to thehydrogen-silicon tetrachloride mixture produces a coating on 0.080"molybdenum wire which has a life of about 0.05 hour at 1700 C.

It is apparent from the foregoing description that the presence of boroneifects a substantial increase in the life of the molybdenum-siliconalloy coatings or layers. The boron-containing coating substantiallyeliminates the common failure of molybdenum-silicon coatings whichsilicon in contact with said heated body until they react with thesurface portions of the body, the molar ratio of silicon halide to boronhalide in said mixture being from about 2.5 :1 to about 18:1, the saidmixture containing hydrogen in an amount of from about 1.5 to about 6times the amount theoretically required to reduce the halides.

sistant to oxidation at elevated temperatures which comprisesheating amolybdenum body to a temperature between about 1600 C. and about 1800C., simultaneously depositing boron and silicon on the surface of thebody by passing a mixture of hydrogen substantially free from Watervapor and oxygen and vapors of boron trichloride and silicontetrachloride over the heated body, and maintaining the deposited boronand silicon in contact with said heated body until they react with thesurface portions of the body, the molar ratio of silicon tetrachlorideto boron trichloride in said mixture being from about 2.521 to about18:1, the said mixture containing hydrogen in an amount of from about1.5 to about 6 times the amount theoretically required to reduce thechlorides.

3. The method of forming refractory metal bodies resistant to oxidationat elevated temperatures which comprises heating a molybdenum body to atemperature between about 1600 C. and about 1800 C., simultaneouslydepositing boron and silicon on the surface of the body by passing amixture of hydrogen substantially free from water vapor and oxygen andvapors of boron trichloride and silicon tetrachloride over the heatedbody, and maintaining the deposited boron and silicon in contact withsaid heated body until they react with the surface portions of the body,the molar ratio of silicon tetrachloride to This type of borontrichloride in said mixture being about 11.911, the said mixturecontaining hydrogen in an amount at least corresponding to thattheoretically required to reduce the chlorides.

4. A method of forming refractory metal bodies resistant to oxidation atelevated temperatures which comprises heating a molybdenum body to atemperture of at least about 1400 C., depositing boron on the surface ofthe body by passing a mixture of hydrogen substantially free from watervapor and oxygen and of vapors of a boron halide over the heated body,depositing silicon on the surface of the body by passing a mixture ofhydrogen substantially free from water vapor and oxygen and of vapors ofa silicon halide over the heated body; the amount of hydrogen present insaid mixtures being at least sufficient to reduce the said halides andthe molar ratios between said silicon halide and said boron halidevarying from about 2.5 :1 to about 18: 1, and maintaining the depositedmaterials in contact with said heated body until they react with thesurface portions of the said body. v

5. A method of forming refractory metal bodies resistant to oxidation atelevated temperatures which comprises heating a molybdenum body to atemperature of at leastabout 1490 C., depositing boron on the surface ofthe body by passing a mixture of hydrogen and of vapors of a boronhalide over the heated body, depositing silicon on thesurface of thebody by passing a mixture of hydrogen and of vapors of a silicon halideover the heated body, the amount of hydrogen present in said mixturesbeing at least suiiicient to reduce the said halides and the molarratios between said silicon halide and said boron halide varying fromabout 2.5 :1 to about 18:1, and maintaining the deposited materials incontact with said heated body until they react with the surface portionsof the said body.

6. A method of forming refractory metal bodies resistant to oxidation atelevated temperatures which comprises heating a molybdenum body to atemperature of at least about 1400 C., depositing boron on the surfaceof the body by passing a mixture of hydrogen substantially free fromwater vapor and oxygen and of vapors of boron trichloride over theheated body, depositing silicon on the surface of the body by passing amixture of hydrogen substantially free from water vapor and oxygen andof vapors of silicon tetrachloride over the heated body, the amount ofhydrogen present in said mixtures being at least sufficient to reducethe said halides and the molar ratios between said silicon halide andsaid boron halide varying from about 2.5:1 to about 18:1, and

maintaining the deposited materials in contact with said heated bodyuntil they react with the surface portions of the said body. 7 V

7. A method of forming refractory metal bodies resistant to oxidation atelevated temperatures which comprises heating a molybdenum body to atemperature of at least about 1400" (1., simultaneously depositing boronand silicon on the surface of the body by passing a mixture of hydrogensubstantially free from water vapor and oxygen and of vapors of a boronhalide and a silicon halide over the heated body, the amount of hydrogenpresent in said mixtures being at least sufficient to reduce the saidhalides and the molar ratios between said silicon halide and said boronhalide varying from about 2.5:1 to about 18:1, and maintaining thedeposited materials in contact with said heated body until they reactwith the surface portions of the said body.

8. A method of forming refractory metal bodies resistant to oxidation atelevated temperatures which comprises heating a molybdenum body to atemperature of at least about 1400" 0., simultaneously depositing boronand silicon on the surface of the body by passing a mixture of hydrogenand of vapors of a boron halide and a silicon halide over the heatedbody, the amount of hy drogen present in said mixture being at leastsufficient to reduce the said halides and the molar ratios between saidsilicon halide and said boron halide varying from about 2.511 to about18:1, and maintaining the deposited materials in contact with saidheated body until they reactwith the surface portionsof the said body.

-9. A method of forming refractory metal bodies resistant to oxidationat elevated temperatures which comprises heatin a molybdenum body to atemperature of at least about 1400 C., simultaneously depositing boronand silicon on the surface of the body by passing a mixture of hydrogensubstantially free from water vapor and oxygen and of vapors of borontrichloride and silicon tetrachloride over the heated body, the amountof hydrogen present in said mixtures being at least sufficient to reducethe said halides and the molar ratios between said silicon halide andsaid boron halide varying from about 2.5:1 to about 18:1, andmaintaining the deposited materials in contact with said heated bodyuntil they react with the surface portions of the said body. I

References Cited in the file of this patent UNITED STATES PATENTS

1. THE METHOD OF FORMING REFRACTORY METAL BODIES RESISTANT TO OXIDATION AT ELEVATED TEMPERATURE WHICH COMPRISES HEATING A MOLYBDENUM BODY TO A TEMPERATURE EXCEEDING ABOUT 1400* C, SIMULTANEOUSLY DEPOSITING BORON AND SILICON ON THE BODY BY PASSING A MIXTURE OF HYDROGEN SUBSTANTIALLY FREE FROM WATER VAPOR AND OXYGEN AND VAPORS OF A BORON HALIDE AND A SLICON HALIDE OVER THE HEATED BODY, AND MAINTAINING THE DEPOSITED BORON AND SILICON IN CONTACT WITH SAID HEATED BODY UNTIL THEY REACT WITH THE SURFACE PORTIONS OF THE BODY, THE MOLAR RATIO OF SILICON HALIDE TO BORON HALIDE IN SAID MIXTURE CONTAINING ABOUT 2.5:1 TO ABOUT 18:1, THE SAID MIXTURE CONTAINING HYDROGEN IN AN AMOUNT OF FROM ABOUT 195 TO ABOUT 6 TIMES THE AMOUNT THEORETICALLY REQUIRED TO REDUCE THE HALIDES. 